Arrangements are known in the state of the art, wherein elements, for example, belts, are arranged in the edge areas of the web and these are used with the aid of suction and/or blowing actions to keep a hold on the web edge in order to eliminate the shrinking effect caused in the cross machine direction as the web is drying. Such arrangements are presented in Printed Patent Specifications SE 440518, 462171, 468217, 517689 and in the published international application WO 90/14467. These known arrangements are rather complicated and one problem in them is that in them the power effect extends over the entire distance of the web in the cross machine direction, whereby it is not possible to control the web's shrinking profile in the cross machine direction with their agency. Thus, with state-of-the-art arrangements the effect is reducing on the entire shrinkage level while at the same time efforts are made to straighten out the edge profiles.
Known in the state of the art is also Printed Patent Specification FI 863614, from which a linearly movable end bearing housing is known.
For spreading out a paper web to the sides to straighten the web such roll solutions are known, wherein the roll surface is provided with oblique grooves on both sides of the roll's central axis. The grooves hereby bring about a spreading effect towards the sides of the paper web, which effect can straighten the web's transfer towards the center and the web's folding.
Published WO Application 97/49863 presents a method and device, where the web is dried and cooled by transporting the web over a blowing device's curved periphery, from which blowing device blowing actions are directed at the web in order to bring about a support zone's backup zone and where moist exhaust air is sucked from the backup zone to inside the blowing device and where edge supports are arranged at each end of the blowing device to support the web and the edges of the support wire supporting the web and where the web is spread out in the cross machine direction by directing the edge supports into an opening angle in relation to the web's direction of travel. In the equipment known from the WO publication, the web is attached at the edges with the aid of special holding wheels provided with suction, that is, with the aid of spreading elements, and the central area of the web is dried with the aid of fixed foil nozzles located below. When located at the beginning of the drying section, such equipment allows wet-spreading of the web, which takes place with the aid of wheels located at the end of the space and provided with suction. The wheels suck and hold the web and the web can be spread out with the aid of the wheels' setting, which is directed outwards. It is a problem with this known solution that the contact-free backup zone does not allow directing of the spreading out. Thus it is an objective of the invention to provide an improvement of this arrangement and to develop it further, whereby the above-mentioned problem will be eliminated.
During all of the history of paper and board making the web's edge areas have posed a problem, in which edge areas shrinkage and rigidity characteristics in the cross machine direction have been inferior to those in the central part of the web. The problem has been especially prominent in single-wire transfer drying sections and it will be a significant factor when making drying sections for high-speed machines, where the above-mentioned smoothness of cross-section profiles is a significant advantage in the quality characteristics.
Controlling the edges is thus a problematic area in papermaking. As single-wire transfer drying sections become more popular, shrinkage and material characteristics in the cross machine direction of the central area have been made quite uniform and owing to this the web is of quite a uniform quality except in the edge areas. In certain cases strong changes, which occur in quality characteristics and in drying-shrinkage and which are concentrating in the edge area, may even be so big that the characteristics of the paper will be so poor that no marketable product is obtained from the web's edge areas, that is, may lead to rejection of customer rolls cut from the edges in the slitting.
As is known from the state of the art, especially in single-wire transfer drying sections of papermaking machines a shrinkage profile having a uniform central part occurs in the web in the cross machine direction and increases strongly in the edge areas. A sharp shrinkage profile at the edges has the result that, for example, in the case of board it is not possible to use the edges in further processing, but the edges are cut off in most cases. This causes production losses and reduces the efficiency of the machine.
Proportioning of the web's fiber orientation to the runnability is a problem in known state-of-the-art solutions. Such a web has the most advantageous runnability, where the fiber orientation is strongly in the machine direction, whereby the distribution ellipse of the fiber orientation is very elongated in the machine direction. However, more fiber orientation in the cross machine direction is a requirement with certain paper grades, and the distribution ellipse describing it thus becomes rounder, for example, in order to ensure the web's dimensional stability, which has led to compromises as regards runnability. Less fiber orientation in the machine direction of fine grade paper, for example, copying paper, which has strengthened the dimensional stability, compared e.g. with newsprint, has caused poorer runnability, which has also been described with the distribution ellipse of the fiber orientation, which is oval for newsprint and rather round for copying paper. In the machine direction, an oval distribution ellipse of the fiber orientation means more shrinkage in the cross machine direction, because there is less fibers in the web's cross machine direction that would prevent shrinkage from taking place in the concerned direction.
The problems known from the state of the art and relating to profile control combined with the drying shrinkage of the edges will be more pronounced along with increasing machine velocities and higher draws used in the machine direction, because the web's edges have a poorer ability to resist the power effects increasing along with the speed, whereby they will stretch and the web has thus to be tightened in the machine direction according to its looser parts, although draws have an adverse effect on the quality characteristics of the paper. As speeds have become higher, it has been necessary to shorten the first drying groups, so that a draw in the machine direction is obtained in the very wet web in early drying by always running later drying groups faster than the preceding one.
It is a problem in known state-of-the-art arrangements when using CD control in the shrinkage control that the control steps are disturbed by the variation of shrinkage. In this regard reference may be made; for example, to the doctor's thesis entitled John Shakespeare: Identification and Control of Cross-machine Profiles in Paper Machines: A Functional Penalty Approach, Tampere University of Technology publications 352, 2001 (ISBN 952-15-0719-5, ISSN 0356-4940).
On the other hand, as regards the state of the art, reference can be made to the measurement and adjustment of the cross machine profile and to the measurement and adjustment solutions for the shrinkage profile and elongation profile or web tension, which are described, for example, in the following publications: FI 990217, FI 992849, U.S. Pat. No. 6,200,422, FI 20020068, U.S. Pat. Nos. 5,943,906 and 6,343,240. The problem in these known solutions is that it is not possible with the aid of these known methods to control the deformation (shrinkage/elongation profile) of the paper sufficiently. On the other hand, a variation of the shrinkage profile also makes the result of several adjustments deteriorate, so that even for this reason it would be worthwhile to maintain both the elongation profile and the shrinkage profile as constant as possible in such a way that the process models used by the adjustment and the real measurement result would correspond with each other. The best performance is hereby obtained from the adjustment.